1. Technical Field of the Invention
The present invention relates to a magnetic resonance imaging method and an apparatus therefor which utilize an NMR (Nuclear Magnetic Resonance) phenomenon, and more particularly, to a magnetic resonance imaging method and an apparatus therefor by means of which high speed two-dimensional and three-dimensional Fourier transformations can be attained.
2. Description of the Prior Art
Referring to FIG. 1, illustrated is a block diagram of a typical magnetic resonance imaging apparatus which is a simplified version of the magnetic resonance imaging apparatus released by A. James and others in "American Journal of Radiology" (pages 1O6 and 206, Vol. 138).
In the figure, the numeral 1 designates a magnet; 2 indicates a body, for instance, a human body lying in a static magnetic field created by the magnet I; 3 denotes a high frequency coil wound around the human body 2; 4 indicates a transmitter/receiver for transmitting an electromagnetic wave to the high frequency coil 3 and receiving an electromagnetic wave (NMR signal) coming from the human body 2; 5 represents a plurality of pairs of gradient magnetic field coils disposed between the magnet 1 and the high frequency coil 3; and 6 is a power source for driving the gradient magnetic field coils 5.
A control circuit generally indicated at 7 serves to control the high frequency coil 3 and the gradient magnetic field coils 5 through the transmitter/receiver 4 as well as through the gradient magnetic field coil power source 6. This control circuit includes sampling means (not illustrated) for obtaining the NMR signal from the human body 2 at a predetermined sampling frequency. The numeral 8 is a computer linked to the control circuit 7. This computer has image reforming means (not illustrated) for obtaining an image on the basis of the received NMR signal. The numeral 9 represents an image display connected to the computer 8.
Next, the operation of a typical magnetic resonance imaging apparatus as depicted in FIG. 1 will be elucidated.
Firstly, the magnetic 1 applies to the human body 2 a uniform static magnetic field in the direction Z, whereby specified atomic nuclei within the human body 2 are irradiated with the electromagnetic wave having the Zeeman energy from a transmitting unit of the transmitter/receiver 4 through the high frequency coil 3. The electromagnetic wave causes the specified atomic nuclei to perform the resonant transition from the ground state to an excited state.
Then the irradiation of electromagnetic wave is halted. Subsequently, the electromagnetic wave emitted from the atomic nuclei within the human body 2 is detected through the high frequency coil 3 by means of a receiving unit of the transmitter/receiver 4. The transmitter/receiver 4 incorporates a receiving A/D converter for receiving the NMR (Nuclear Magnetic Resonance) signal from the high frequency coil 3 in accordance with the predetermined sampling frequency.
In this case, a slope is given to the static magnetic field by use of the gradient magnetic field coils 5, thereby deciding from which positions of the human body 2 the signal is coming. In practice, the gradient magnetic field coils 5 are composed of three pairs of coils for generating the first gradient magnetic field Gz, the second gradient magnetic field Gy, and the third gradient magnetic field Gx in three orthogonal axial directions Z, Y and X, respectively.
On the other hand, the computer 8 serves to control both the power source 6 for supplying an electric current to the gradient magnetic coils 5 and the transmitter/receiver 4 through the control circuit 7, and also permits the image display 9 to display a resultant image obtained in accordance with the high speed Fourier transformation. It is to be noted that details of the magnetic resonance imaging method based on the Fourier transformation are disclosed in, for instance, British Pat. No. 2,079,946, and hence a detailed description is not given herein.
A conventional magnetic resonance imaging method performed in association with the two-dimensional Fourier transformation using the magnetic resonance imaging apparatus depicted in FIG. 1 will now be explained.
Referring to FIG. 2, a pulse sequence diagram is shown that explains the prior art magnetic resonance imaging method (a FLASH method) as described in, e.g., the magazine "Magnetic Resonance Imaging" (page 297, Vol. 3). The reference symbol RF indicates a high frequency magnetic field pulse generated from the high frequency coil 3 on the basis of the pulse transmitted from the transmitter/receiver 4; Gz, Gy and Gx denote the first, second and third gradient magnetic fields, respectively, each corresponding to one of the three orthogonal axial directions Z, Y and X; and S represents an NMR signal, i.e., a spin echo signal received by the transmitter/receiver 4 through the high frequency coil 3.